The present invention relates to a test method and apparatus for measuring capacitance and leakage current and more particularly relates to apparatus useful in finding isolated net shorts, opens, and leakage in unpopulated multilayer boards or substrates with voltage and ground reference planes.
Testing of unpopulated multilayer boards and substrates for shorts and opens in signal paths results in improved product reliability. A short is an electrical continuity where it is not desired often caused by excessive copper between two circuit lines after etching. An open is a break in a circuit line preventing circuit continuity. Typically, testing is done with a "bed-of-nails" test fixture. The nails are probes typically spaced 0.100 inches apart which make contact electrically between the tester and the points of contact on the board being tested. It is highly desirable to contact all test points at once to avoid step and repeat operations. The difficulty with the bed-of-nails approach are problems with the probe heads which present design, manufacture, and maintenance challenges and particularly with the decreasing size of surface features and grid spacings on boards being produced. Construction of a bed-of-nails test head for each of the different types of boards and substrates to be checked involves a high cost and long fabrication time.
An alternative to the bed-of-nails approach for testing for shorts and opens is to measure the net capacitance relative to an internal or external reference plane.
Measurements are typically done with an AC capacitance meter applying a low voltage/high frequency test stimulus. The capacitive response decreases as the series resistance increases thus limiting the ability to detect leakage paths.
It is an object of the present invention to provide a capacitance and leakage test apparatus which performs a voltage stress test while measuring capacitance.
It is another object of the present invention to provide a capacitance and leakage test apparatus which provides extended leakage response beyond the range of conventional sine wave capacitance meters.
It is a further object of the present invention to provide a capacitance and leakage test apparatus which will detect high resistance leakage paths between nets while measuring capacitance.
It is a still further object of the present invention to provide a capacitance and leakage test apparatus which will detect too-closely spaced signal nets while measuring capacitance.